The present invention generally relates to coatings capable of use on components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to methods and a system for applying a thermal barrier coating (TBC) with improved erosion resistance by intermediately removing overspray byproduct that accumulates during the application process.
The use of thermal barrier coatings (TBCs) on components such as combustors, high pressure turbine (HPT) blades, vanes and shrouds is increasing in commercial as well as military gas turbine engines. The thermal insulation provided by a TBC enables such components to survive higher operating temperatures, increases component durability, and improves engine reliability. TBCs are typically formed of a ceramic material and deposited on an environmentally-protective bond coat to form what is termed a TBC system. Bond coats are typically formed of an oxidation-resistant diffusion coating such as diffusion aluminide, platinum aluminide or an oxidation-resistant overlay coating such as MCrAlY (where M is iron, cobalt and/or nickel).
Various processes can be used to deposit TBC materials, including thermal spray processes such as air plasma spraying (APS), vacuum plasma spraying (VPS), low pressure plasma spraying (LPPS), and suspension plasma spraying (SPS). However, these spray processes may experience problems with overspray wherein the TBC materials are deposited on undesired surfaces of the coated component to form what is hereinafter referred to as an overspray byproduct. The overspray byproduct is only loosely adherent and is highly undesirable from the viewpoint of mechanical robustness, erosion resistance and thermal spalling resistance. This problem can be observed in SPS processes that use a feedstock comprising fine particles suspended in a liquid agent. The suspension is fed to a plasma spray torch in a controlled manner and injected into the plasma plume for deposition onto a substrate. The particles typically, but not necessarily, have a median diameter in the range about 0.4 micrometers to about 2 micrometers, which may be significantly smaller than powder media typically used with other conventional thermal spray processes. The liquid agent typically is a solution of water, alcohol, or similar solvent mixed with an additive, for example, ethanol at about 10 percent by weight, using polyethyleneimine as a dispersant (at 0.2 percent by weight of the solids). In a typical SPS process, the plasma spray torch motion and spraying routine are traditionally programmed to provide the desired thickness distribution in the resultant coating without regard for overspray byproduct build up.
A vane segment 10 of a gas turbine engine is represented in FIG. 1 for purposes of the following discussion. The segment 10 comprises several airfoils 12 connected to outer and inner bands 14 and 16. When SPS is performed on, for example, one of the airfoils 12, the byproduct accumulates on the surfaces of the inner and outer bands 16 and 14 and the fillets where the airfoil 12 is joined to the bands 14 and 16. If the entire segment 10 sprayed in one uninterrupted program, as is a common procedure, the overspray byproduct is subsequently entrapped by the sprayed coating intentionally deposited on the bands 14 and 16. It has been determined that the entrapped overspray byproduct results in a softer region of the coating that may have lower erosion resistance and be prone to spallation. The decreased erosion resistance and greater susceptibility to thermal spalling are attributed to the inadequate adherence and non-uniform thickness of the overspray byproduct.
As further explanation, an SPS process is represented in FIGS. 2 through 5. FIG. 2 schematically represents an uncoated vane segment 10 viewed from a side. In FIG. 3, an SPS plasma spray torch 18 is represented as spraying ceramic material onto the segment 10 to deposit a TBC coating 20 on portions of the outer band 14 and airfoil 12. During spraying, overspray byproduct 22 accumulates over a portion of the inner band 16 and an adjacent portion of the airfoil 12. In FIG. 4, the SPS process is continued as ceramic material is deposited to form the coating 20 on portions of the inner band 16 and airfoil 12, including the overspray byproduct 22. Finally, in FIG. 5, the ceramic material is sprayed onto remaining portions of the airfoil 12 of the segment 10. The resulting coated segment 10 may be prone to erosion and spalling in the region of the coating 20 where the overspray byproduct 22 was deposited for the reasons described above. It should be understood that the overspray byproduct 22 can accumulate not only on the regions that have not seen any TBC deposit but also in regions on top of the TBC 20 that was already deposited. The layer of overspray byproduct 22, which is often much thinner than TBC 20, on top of an already accomplished TBC 20 is not shown in any of the figures.
In previous methods of TBC deposition, the overspray byproduct 22 is either tolerated or regions prone to overspray byproduct 22 were covered prior to the spraying process with a material such as a barrier tape, cover, or mask. Continuing the deposition process while retaining the overspray by product reduces the robustness of the TBC 20. While covering the overspray-affected regions prior to the spraying process can be an effective method of avoiding the problem, it can be difficult to efficiently implement such a method into a continuous fabrication process. Another disadvantage is that it is difficult to select materials that can be utilized for purposes of covering the potential overspray regions which also have thermal stability at the temperatures involved in the thermal spray processes.
Accordingly, there is a need for a method of applying TBCs to components that is capable of avoiding or limiting the problems associated with overspray byproduct buildup. A need exists to remove the byproduct in such a way that the process lends itself to an efficient continuous coating operation resulting in increased throughput.